I think its a serious bug. I ordered one plasteel longsword, three dudes started one, one got finished, now i have 240 plasteel tied up in two unfinishable projects. No workaround seems to exist after an order is finished.
You should be able to click on the unfinished longswords and click "cancel" to get most of the resources back (some are still wasted).
To try to avoid the bug, try to make sure the crafter/artist continues work on the thing often, instead of doing other things. I've only had the bug happen when the crafter/artist left for a while (mentally broke, went out to shoot at centipedes for a while, whatever).
Also, keep only a few people set to crafting or art. I have only two crafters, and I manage art in the overview, only giving anyone the art job (at higher priority than most things, to make sure they work on it constantly to avoid the bug and to get it done) when I specifically want them to make a sculpture, then turning it off once they finish.